Modulating innate immune activity by agonizing or antagonizing pattern recognition receptors (PRRs) has vast potential for clinical applications both as monotherapy and in combination with other pharmaceutical or bio-pharmaceutical agents. The applications range from alleviating autoimmune disorders through immunosuppression to treating solid and hematological cancers by stimulating innate anti-tumor immunity, as well as uses in anti-viral therapy or as vaccine adjuvant.
One of the PRRs involved in the effective activation of antigen presenting cells (APCs) is the stimulator of interferon genes (STING) protein. STING is an evolutionarily conserved, cytosolic PRR that is part of the cGAS-CDN-STING axis. Aberrant dsDNA in the cytosol, as a consequence of cell stress, viral or intracellular bacterial infection, failed mitosis, or phagocytosis, is recognized by the cGAS enzyme, which synthesizes the non-canonical cyclic di-nucleotide (CDN), 2′3′cGAMP. 2′3′cGAMP binds to and stabilizes the STING dimer, resulting in IRF3 and NFkB activation and synthesis of type I interferon. STING protein plays an important role in innate cellular responses to viral infection and aberrant cytosolic DNA accumulation in both target cells and responding innate immune cells. The pleiotropic effects of STING activation are cell-type and context dependent. For example, overstimulation of T cells and B cells through STING leads to a pro-apoptotic phenotype, while in myeloid cells STING activation elevates type I IFN and pro-inflammatory cytokines without an increase in apoptosis.
Tumor derived dsDNA is phagocytosed by resident dendritic cells (DCs), which stimulates the cGAS-CDN-STING axis and activates DCs, leading to lymph node migration and, ultimately, proliferation of antigen specific CD4+ and CD8+ T cells. This process and the accompanying type I IFN response are often absent in tumors that lack a T cell infiltrate, highlighting the potential of STING agonization to directly address the mechanism of escape exploited by these tumors. Although several CDN derived ligands have shown pre-clinical promise as STING agonists, their relatively large molecular weight and polarity have limited their application to intratumoral injection. Moreover, binding and activation/inhibition of the cytosolic STING protein is limited in vivo by cell membrane permeability. Furthermore, despite their therapeutic efficacy via systemic administration (i.v. or i.p.), the previously identified small molecule STING agonists DMXAA and CMA exhibit species selectivity, prohibiting their use as human therapeutics. Thus, there is a need for human-active small molecule modulators of STING for use as effective therapeutic agents. The present application addresses this need. The novel compounds of this application overcome the limitations of CDN derived ligands.